Treatment of 5-day cyclic rats with 10 or 100 microgram estradiol benzoate on the day of estrus induced a luteal phase in all animals studied. On the other hand, an injection with 1 microgram estradiol benzoate given on the same day failed to induce pseudopregnancy. When 10 mg progesterone were injected on the day of estrus, about 50% of the rats became pseudopregnant, whereas most of the remaining rats had a 6-day cycle. The injection of 10 or 100 microgram estradiol benzoate or 10 mg progesterone induced a period of increased PRL secretion which lasted for 2--4 days, followed by twice daily surges of PRL at the end of the dark and light periods, respectively. It is argued that pseudopregnancy induction by estradiol benzoate or progesterone is primarily a result of the induction of a period of increased PRL secretion. In this way, progesterone secretion by the recently formed corpora lutea is induced, and the elevated levels of progesterone in turn generate diurnal surges of PRL.